In many applications using supercritical fluids, the fluid has to be depressurized after extraction or chromatography. A restrictor is typically used to restrict the flow and/or depressurize the fluid. There are a number of different kinds of restrictors and some of them are: a) a plain tube that is crimped, b) a tube that is heated and tapered to reduce the inside diameter (Chester, T. L., Innis, D. P. and Owens, G. D., Anal Chem., 57, 1985, 2243-2247), c) an integral restrictor: where you melt the end of tubing to close it shut and then grind it to give the proper size hole (Guthrie, E. J. and Schwatz, H. E., J. Chromatographic Sci., 24, 1986, 236-241), d) a metal orifice restrictor where a laser drilled hole serves as the restrictor (Randall, L. G. and Wahrfartig, A. L., Anal. Chem. 50, 1978, 1703-1705), and e) a valve (Sin, C. H. et al., Anal. Chem., 58, 1986, 487-490). When a fluid such as carbon dioxide depressurizes, it cools and sometimes solidifies, plugging the restrictor (Dick, R. D., et al., Anal. Chem., 58, 1986, 2057-2064). To alleviate this situation, heat is usually supplied through an external device. This is usually achieved through a resistive heating tape or cartridge near the restrictor.
These current restrictor technologies have a number of problems. A crimped tube cannot be manufactured reproducibility and can plug easily (Smith, R. D., Fjeldsted, J. C., and Lee, M. L., Chromatogr. 247, 1982, 241-243). A tapered tube does not plug as easily, however, it is difficult to manufacture and very fragile (Smith, R. D., et al., Anal. Chem., 55, 1983, 2266-2272) and since they are typically made out of fused silica, they are poor conductors of heat. Similar to the tapered restrictors, the integral restrictors are poor conductors of heat and plug often (Wright, B. W. and Smith, R. D., Modern Supercritical Fluid Chromatography, Ed. White, C. M., Huthig, 1988, 189-210). A metal orifice restrictor offers easy replacement, but the flow varies depending how it is assembled (Dick, R. D., et al., Anal. Chem., 58, 1986, 2057-2064). Further, the temperature control algorithm causes changes in flow rate due to the high thermal expansion. With respect to a valve, it has a high dead volume which reduces the efficiency and accuracy of the system.
In supercritical fluid extraction, the restrictors are used to collect the soluble analytes for further processing. Typically, a solvent is used for collecting the soluble analytes. Heating the restrictor is necessary to stop it from plugging during operation. Heating the restrictor by heating the solvent is not a viable solution since the low boiling point of the solvent leads to vaporization of the solvent and therefore inaccuracies in the result. Therefore, there is a need for heating the restrictor non-invasively. Further, a restrictor is needed which is comprised of a hard material that will not erode easily and will not change shape easily under pressure and has a low coefficient of thermal expansion and a high thermal conductivity.